Inorganic microporous materials, such as zeolites, metal oxide zeotype solids, and layered silicates are of considerable technological importance due to their applications in adsorption, separation, catalysis, and ion exchange. Many of these applications utilize the inorganic microporous material in composites, such as nanocomposites.
Zeolites are built of tetrahedral SiO4 and AlO4, while mixed oxide zeolitic materials, e.g., titanosilicates, consist of tetrahedral SiO4 and octahedral MgO6. Zeolites and zeotype materials are framework materials—materials that contain one-, two-, or three-dimensional open channels whose dimension is on the order of the size of the molecules. This microporous structure provides the basis for many of the aforementioned applications.
On the other hand, layered silicates are a class of inorganic materials that are naturally layered in structure. Layered silicates refer both to natural clays, like smectites, and to synthetic layered silicates such as magadiite and mica. Among natural clays, both montmorillonite and hectorite belong to the smectite family and are the most commonly used layered silicates in nanocomposites due to their high cation-exchange capacities, surface areas, surface reactivities, and adsorptive properties (see, e.g., Carrado, K. A., “Synthetic organo- and polymer-clays: preparation, characterization, and materials applications.” Applied Clay Science, 17, 1–23 (2000)). Synthetic layered silicates such as magadiite have also been used successfully in the synthesis of nanocomposites (see Wang, Z., Pinnavaia, T. J., “Hybrid Organic-Inorganic Nanocomposites: Exfoliation of Magadiite Nanolayers in an Elastomeric Epoxy Polymer,” Chem. Mater. 10 1820–1826 (1998)).
The structure of typical layered silicates consists of silicate layers with aluminum octahedra, and cations between the layers to satisfy overall charge balance. For example, the crystal structure of montmorillonite consists of two fused silica tetrahedral sheets sandwiching an edge-shared octahedral sheet of either aluminum or magnesium hydroxide as shown in FIG. 1 (see, e.g., Kornmann, X., Synthesis and Characterization of Thermoset-Clay Nanocomposites, Ph.D. Thesis, Division of Polymer Engineering. Lulea, Lulea University of Technology (2000)). As in zeolites, isomorphous substitution of [SiO4] tetrahedra with [AlO4]− tetrahedra and of [AlO6]3− octahedra with [MgO6]4− octahedra in the sheet causes an excess of negative charges within the layers. These net negative charges are balanced with additional cations, such as Ca2+ and Na+, located between the layers. Considerable numbers of water molecules are coordinated to these cations. These coordination bonds account for the high hydrophillicity of montmorillonite. The thickness of the layer of montmorillonite is about 1 nm, while its aspect ratio is very high, typically in the range of 100–1000 (see, e.g., Krishnamoorti, R., Vaia, R. A., Giannelis, E. P., “Structure and Dynamics of Polymer-Layered Silicate Nanocomposites,” Chem. Mater. 8, 1728–1734 (1996)).
Conventional layered materials, e.g., montmorillonite, do not possess channel systems or open frameworks within the layers. The absence of channels and/or open frameworks limits their use in applications like catalysis, adsorption, and separation. Extensive efforts have been devoted to exfoliate layered silicates to make mesoporous materials. For example, Carrado et al. reported the synthesis of new mesoporous materials from hectorite by the removal of a polymer template used as a pore structure-directing agent (see Carrado, K. A., “Synthetic organo- and polymer-clays: preparation, characterization, and materials applications,” Applied Clay Science, 17 1–23 (2000)). These new porous materials were successfully tested for their use as potential catalysts and catalytic supports.
An example of a layered silicate material having channels in the silicate layers is MCM-22(P). MCM-22 is a microporous aluminum silicate first reported by Mobil researchers (see Leonowicz, M. E., Lawton, J. A., Lawton, S. L., Rubin, M. K., “MCM-22: A Molecular Sieve with Two Independent Multidimensional Channel Systems,” Science, 264, 1910–1913 (1994)). These researchers hydrothermally synthesized this material from a layered aluminum silicate precursor, which they call MCM-22(P), with hexamethyleneimine (HMI) as a structure-directing agent. Upon calcination of MCM-22(P), the thermally stable open framework MCM-22 was formed. A proposed structure of MCM-22(P), shown in FIG. 2, includes aluminum silicate layers weakly linked together with HMI along the [0 0 1] direction. The layers of MCM-22(P) consist of a hexagonal array of hourglass pockets on the [0 0 1] planes having 12-membered ring (12MR) apertures on both sides of the layers. Two-dimensional channels run in the plane of the MCM-22(P) layers. However, molecules cannot penetrate along the c-axis (perpendicular to the layers), due to the small 6-membered ring (6MR) necks, i.e., rings defined by six [SiO4] tetrahedra. Thus, there is no channel system between the layers along the c-axis, i.e., no micropores perpendicular to the layer short dimension. FIG. 3(a) and FIG. 3(b) schematically show the structure of a layer of a layered silicate with no channels and with channels in the plane of the layer, respectively.
In 1998, Corma and coworkers delaminated MCM-22(P), to make a new aluminosilicate (ITQ-2) with zeolite-type catalytic sites within thin sheets (see Corma, A., Fones, V., Pergher, S. B., Maesen, Th. L. M., Buglass, J. G., “Delaminated zeolite precursors as selective acidic catalysts,” Nature, 396, 353–356 (1998)).
Layered silicates are promising candidates for use in composites. For example, polymer-layered silicate (PLS) nanocomposites are of great scientific and industrial interest. In the early 1990's, researchers from Toyota showed a possibility to fabricate unprecedented nanostructured materials with polymer and layered silicates (see Yano, K., Usuki, A., Okada, A., “Synthesis and properties of polyimide-clay hybrid films,” J. Polym. Sci. Part, A 35, 2289–2294 (1997)). These researchers demonstrated nylon-based nanocomposites with layered silicates that exhibit dramatic improvements in mechanical, barrier and thermal properties with as little as 2 vol. % of layered silicates. Since then, this area has drawn considerable research efforts.